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This transponder includes a dielectric support, an antenna and a covering layer. The central portion of the antenna forms a self induction loop which, together with the junction capacity of a non-linear component serving to close the loop, provides a circuit resonating at the frequency at which the

This transponder includes a dielectric support, an antenna and a covering layer. The central portion of the antenna forms a self induction loop which, together with the junction capacity of a non-linear component serving to close the loop, provides a circuit resonating at the frequency at which the transponder receives its energy. The antenna radiates energy at double this frequency which energy may be detected by a receiver.

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A passive transponder for seeking out persons by means of a combined transmitter-receiver capable of transmitting at a first frequency and receiving at a second frequency which is a harmonic of said first frequency, comprising: a dielectric support (11), an antenna formed from a thin conductive shee

A passive transponder for seeking out persons by means of a combined transmitter-receiver capable of transmitting at a first frequency and receiving at a second frequency which is a harmonic of said first frequency, comprising: a dielectric support (11), an antenna formed from a thin conductive sheet (12) fixed to said support and including a non-linear passive component (19) and a conductive portion (16) electrically parallel to said passive component to establish a direct current bias thereof, said antenna sheet defining a cutout portion (15) having an edge (17) which defines a conductive path closed by the passive component to form a self-inductive loop, said loop, together with the capacity of the passive component, providing a circuit resonating at said first frequency, wherein the antenna sheet is linearly elongate, and the cutout portion is disposed proximate a central portion of said elongate antenna sheet intermediate opposite ends thereof.